A conventional electrophotographic process comprises the steps of uniformly charging a photoconductive insulating layer, exposing this layer, eliminating an electric charge from the exposed portion to form an electric latent image, applying a colored, charged, fine powder called a toner to the latent image to visualize the same (developing step), transferring the obtained visible image onto a transfer material such as a transfer paper (transfer step) and permanently fixing the image by heating, compression or another suitable fixing method (fixing step) as described in U.S. Pat. Nos. 2,297,691 and 2,357,809.
Thus, the toner must have functions required not only in the developing step but also in the transferring and fixing steps.
Generally, a toner is deteriorated by mechanical friction due to shearing or impact force applied thereto while thousands or more of copies are produced by a mechanical treatment carried out in a developing device. Though the problem of the deterioration of the toner can be solved by using a high-molecular tough resin resistant to the mechanical friction, this type of resin generally has a high softening point and, therefore, sufficient fixing is impossible by a non-contact fixing method such as an oven fixing method or radiant fixing method with infrared rays due to poor thermal efficiency thereof. Further, when the resin is used in a contact fixing method such as a usual heat roller fixing method, a high temperature is required of the heat roller so as to effect the fixing sufficiently and, therefore, problems such as damage to the fixing device, curling of the paper an increase in energy consumption are present in this method, since a high thermal efficiency is obtained and, in addition, when this type of resin is used, the production efficiency of the toner by finely pulverizing the resin is reduced seriously. Thus, a binder resin having an excessively high degree of polymerization and an excessively high softening point cannot be used. The heat roller fixing method has been employed widely, ranging from low-speed to high-speed fixing systems, because a remarkably high thermal efficiency is obtained as the surface of the heat roller is pressed against a toner image surface of the sheet to be fixed. However, a so-called offset phenomenon is caused when the heat roller is brought into contact with the toner image surface, since the toner adheres to the surface of the heat roller and it is then transferred onto the subsequent transfer paper. To prevent this phenomenon, the surface of the heat roller is treated with a material having excellent releasing properties such as a fluororesin and, in addition, a releasing agent such as silicone oil is applied to the surface of the heat roller to inhibit the offset phenomenon completely.
However, this process in which silicone oil, etc. are used is not preferred, since it necessitates a large fixing device to cause cost increases and a complicated operation.
Though a process for preventing the offset phenomenon by widening the molecular weight distribution of the binder resin is disclosed in Japanese Patent Publication No. 6895/1980 and Laid-Open No. 98202/1981, the degree of polymerization of the resin is increased and a high fixing temperature is necessitated in this process.
Though a further improved process for preventing the offset phenomenon by asymmetrizing or crosslinking the resin is disclosed in Japanese Patent Publication No. 493/1982 and Laid-Open Nos. 44836/1975 and 37353/1982, the problem of the fixing temperature has not yet been solved.
The practical operation temperature ranges from the minimum fixing temperature to a hot offset temperature, since the minimum fixing temperature lies between a cold offset temperature and the hot offset temperature. Therefore, by lowering the minimum fixing temperature as far as possible and elevating the minimum hot offset temperature as far as possible, the practical fixing temperature can be lowered and the practical operation temperature range can be widened. In addition, energy saving, fixing at a high speed and prevention of curling of the paper can be attained. Further, double copying can be effected by this technique without any problems to obtain advantages such as an intelligent copier, mitigation of the accuracy, and temperature control of the fixing device.
Resins and toners having excellent fixing properties an offset resistance are always in demand.
Though it is known that when a styrene binder resin is used a paraffin wax, a low-molecular polyolefin or the like is used as an offset inhibitor for attaining the above-mentioned purposes as disclosed in Japanese Patent Laid-Open Nos. 65232/1974, 28840/1975 and 81342/1975, this process also has problems in that the intended effect cannot be obtained when the amount of the offset inhibitor is insufficient and the deterioration of the developer is accelerated when the amount is excessive. When a polyester resin is used, the offset inhibitor is substantially ineffective and when it is used in a large amount, the deterioration of the developer is also accelerated.
Polyester resins have essentially excellent fixing properties and sufficient fixing is possible even by a non-contact fixing process as disclosed in U.S. Pat. No. 3,590,000, but they could not be used easily in the heat roller fixing process because of the offset phenomenon. As described in Japanese Patent Laid-Open Nos. 44836/1975, 37353/1982 and 109875/1982, even when the offset resistance of the polyester resin is improved with a polybasic carboxylic acid, the offset resistance is still insufficient for practical use or, even if it is sufficient, an inherent low-temperature fixing property of the polyester resin is sacrifice. The pulverizability thereof is the toner-forming step is very poor and problems are posed also in the production of the developing agent.